1. Field of the Invention
The present invention relates to an objective optical system, optical pickup apparatus and optical information recording/reproducing apparatus.
2. Description of the Related Art
In recent years, in an optical pickup apparatus, there has been a growing tendency for adopting a short wavelength laser light source as a laser light source for reproducing the information recorded on an optical disk and for recording the information on an optical disk. For example, there have been efforts made for commercial use of a laser light source having a wavelength of 405 nm such as in a blue-violet semiconductor laser and blue-violet SHG laser for converting the wavelength of an infrared semiconductor laser using the second harmonic wave.
When employing an objective lens having the same numerical aperture (NA) as that of the DVD (Digital Versatile Disk), use of such a blue-violet laser light source allows recording the information of 15-20 GB on an optical disk having a diameter of 12 cm. When the numerical aperture (NA) of the objective lens is increased to 0.85, it permits recording the information of 23-25 GB on the optical disk having a diameter of 12 cm. In the following description of this specification, the optical disk and magnetic optical disk using a blue-violet laser light source will be collectively called the high-density optical disk.
Although, the appropriate recording/reproducing (recording and/or reproducing) of the information is said to be performed using such a high-density optical disk, the value of an optical disk player/recorder as a product cannot be said to be sufficient. In the following description, “recording/reproducing” refers to recording and/or reproducing.
Based on the fact that the DVDs (Digital Versatile Disc) and CDs (Compact Disk) recording a great variety of information are being marketed at present, the mere recording/reproducing of information using a high-density optical disk is not sufficient. For example, the capability of similarly appropriate recording/reproducing of information using the DVDs and CDs owned by a user amounts to raising of the product value of an optical disk player/recorder for a high-density optical disk. Against this backdrop, an optical pickup apparatus mounted on the optical disk player/recorder for a high-density optical disk is preferred to have performances of ensuring the appropriate recording/reproducing of information, while maintaining the compatibility among three types of optical disks—high-density optical disk, DVD and CD.
One of the methods for ensuring appropriate recording/reproducing of information while maintaining the compatibility among the high-density optical disk, DVD and CD is to provide the optical system for high-density optical disk and the optical system for DVD and CD separately so that switching will be made between these two systems in response to the recording density of the optical disk for recording/reproducing of information. This method, however, requires a plurality of optical systems. This is disadvantageous for reduction of physical size and production costs.
Thus, to simplify the configuration of the optical pickup apparatus and to reduce the cost, it is preferred in an optical pickup apparatus having compatibility to provide commonality between the optical system for high-density optical disk and optical system for DVD and CD, and to minimize the number of optical parts constituting the optical pickup apparatus. Simplification of the configuration of the optical pickup apparatus and cost cutting are enhanced by providing commonality of the objective optical system arranged opposite to the optical disk. To get the objective optical system that can be used in common for a plurality of the optical disks, having different wavelengths of light flux, to be used for recording/reproducing, it is necessary to form in an objective optical system a phase structure where spherical aberration depends on wavelength.
In this case, however, the wavelengths of the light flux used in each optical disk and the thicknesses of the protective substrate are different. This makes it difficult to form a satisfactorily aberration-corrected light condensed spot on the surface of the optical disk where information is recorded. Thus, the known art of correcting the aberration is to provide a diffraction structure on the optical surface of the objective lens that is a constituent element of the optical pickup apparatus. (For example, see Patent Document 1: Official Gazette of Japanese Patent Tokkai 2002-298422).
However, the art disclosed in Patent Document 1 for aberration correction is configured in such a structure that a diffraction structure is provided on one surface of the objective lens. It provides a sufficient spherical aberration correcting function for recording/reproducing using two types of the optical disks using different wavelengths and/or having different protective substrate thicknesses. However, the following problem must be solved before a sufficient compatibility is achieved among three types of optical disks.
The wavelengths of the light fluxes used in the high-density optical disk, DVD and CD are λ1=about 400 nm, λ2=about 655 nm and λ3=about 785 nm, respectively. Since λ1:λ3≈1:2, the ratio of the diffraction order providing the maximum diffraction efficiency is λ1:λ3=2:1 in the blazed diffraction structure. (For example, when the order of diffraction providing the maximum diffraction efficiency in relation to the light flux of λ1 is 6, the order of diffraction providing the maximum diffraction efficiency in relation to the light flux of λ3 is 3).
Further, the effect of diffraction is determined by the difference between the value of wavelength of a diffracted light times the diffraction order of the diffracted light and the value of wavelength of an other diffracted light times the diffraction order of the other diffracted light, and by the pitch of the diffraction. Therefore, the value, (λ1×2)−(λ3×1), is reduced when the ratio of the diffraction order of λ1 and λ3 is 2 to 1. Thus, when the diffraction structure is designed with the blazed wavelength as the value close to even number times the wavelength λ1, for example, then there will be a reduced mutual diffraction in the light flux of the wavelength λ1 and that of λ3. This will result in difficult compatibility between the high-density optical disk and CD.
To be more specific, even if the difference between the value of wavelength of a diffracted light times the diffraction order of the diffracted light and the value of wavelength of an other diffracted light times the diffraction order of the other diffracted light is small, it is theoretically possible to achieve compatibility by using a small diffraction. In this case, however, the pitch of the ring-shaped zones of the diffraction structure must be reduced. If the pitch of the ring-shaped zones of the diffraction structure is reduced, it will become difficult to fabricate an optical element such as a lens and there will be a reduction in the amount of the transmitted light of the fabricated optical element. Further, a large aberration will be caused by the variation of the wavelength within the minute range of several nanometers due to the variation in the laser light source output and others. Such problems will be left unsolved.